recursively find subsets

Question

Here is a recursive function that I\'m trying to create that finds all the subsets passed in an STL set. the two params are an STL set to search for subjects, and a number i >=

Answer 1

From what I understand you are actually trying to generate all subsets of 'i' elements from a given set right ?

Modifying the input set is going to get you into trouble, you'd be better off not modifying it.

I think that the idea is simple enough, though I would say that you got it backwards. Since it looks like homework, i won't give you a C++ algorithm ;)

generate_subsets(set, sizeOfSubsets) # I assume sizeOfSubsets cannot be negative
                                     # use a type that enforces this for god's sake!
  if sizeOfSubsets is 0 then return {}
  else if sizeOfSubsets is 1 then
    result = []
    for each element in set do result <- result + {element}
    return result
  else
    result = []
    baseSubsets = generate_subsets(set, sizeOfSubsets - 1)
    for each subset in baseSubssets
      for each element in set
        if no element in subset then result <- result + { subset + element }
    return result

The key points are:

• generate the subsets of lower rank first, as you'll have to iterate over them
• don't try to insert an element in a subset if it already is, it would give you a subset of incorrect size

Now, you'll have to understand this and transpose it to 'real' code.